What Probiotics can do for you?
The Science
The Studies
"At The Naked Pharmacy, we ensure that all of our supplements are made with effective strength bioactives, so they are clinically proven to work.
We aim for each supplement to be scientifically supported by multiple randomised placebo-controlled studies. All clinical trial studies we use to support our supplements are undertaken on human patients, using the same dosage and formulation of the product. The scientific studies are published in peer review journals."
Saccharomyces Boulardii
L. V. McFarland et al., JAMA, 1994. Double-blind randomised placebo-controlled trial, 124 human participants.
What the study was about:
This trial tested whether adding S. boulardii to standard antibiotic therapy (metronidazole or vancomycin) for active C. difficile disease would reduce recurrence and improve outcomes.
How they did it:
Patients with active C. difficile received standard antibiotics; after 4 days of continuous antibiotics, they were randomised to receive S. boulardii 500 mg twice daily or placebo for 4 weeks, and were followed for 4 additional weeks.
What they found:
The S. boulardii group showed a lower rate of recurrence in certain subgroups (notably those who received vancomycin), suggesting a beneficial adjunctive effect in preventing recurrence for some patients.
What it means:
Adding S. boulardii to antibiotic therapy may reduce recurrence of C. difficile infection in selected patients, particularly when given with vancomycin — supporting its use as an adjunct in clinical practice for recurrence prevention in some contexts. PMCU.S. Pharmacist
(Authors; PubMed 17669103), 2007. Randomised, double-blind, placebo-controlled trial, adults undergoing H. pylori eradication therapy.
What the study was about:
To evaluate whether adding S. boulardii to standard 14-day triple therapy for H. pylori reduces antibiotic-associated side effects (diarrhoea, dyspepsia) and affects eradication success.
How they did it:
Adults receiving a 14-day triple anti-H. pylori therapy were randomised to adjunctive S. boulardii or placebo; outcomes included side-effect rates, tolerability, and eradication rates.
What they found:
Saccharomyces boulardii reduced the incidence of antibiotic-associated diarrhoea and improved treatment tolerability; it did not significantly increase H. pylori eradication rates in most trials, though it consistently lowered adverse GI effects. Safety was acceptable.
What it means:
When used alongside H. pylori eradication regimens, S. boulardii can reduce treatment-related GI side effects and improve tolerability — useful clinically to improve completion rates of eradication therapy.
Lactobacillus Reuteri
R. Mobini et al., Diabetes, Obesity and Metabolism, 2017. Double-blind randomised placebo-controlled trial, 46 adult participants with type 2 diabetes on insulin therapy.
What the study was about:
This trial tested whether 12 weeks of oral supplementation with L. reuteri DSM 17938 could improve glycaemic control (HbA1c) and related metabolic measures in adults with established type 2 diabetes.
How they did it:
46 adults were randomised to receive placebo, low-dose (≈10^8 CFU/day) or high-dose (≈10^10 CFU/day) L. reuteri DSM 17938 for 12 weeks. Primary outcome was change in HbA1c; secondary outcomes included insulin sensitivity (glucose clamp), liver fat, body composition, faecal microbiota and serum bile acids.
What they found:
Overall, HbA1c did not differ significantly between groups. However, the high-dose group showed improved insulin sensitivity in a subset of participants. Responders tended to have higher baseline gut microbial diversity; responders also showed increases in specific secondary bile acids correlated with improved insulin sensitivity.
What it means:
While L. reuteri DSM 17938 did not lower HbA1c across the whole cohort, it produced metabolic benefits (improved insulin sensitivity) in a subset of patients — suggesting that host microbiota composition and dose matter when translating probiotic effects to metabolic outcomes. PubMed
Study: Intake of Lactobacillus reuteri improves incretin and insulin secretion in glucose-tolerant humans — a proof-of-concept.
M.-C. Simon et al., Diabetes Care, 2015. Randomised (proof-of-concept) clinical trial, healthy/glucose-tolerant adult volunteers.
What the study was about:
This study investigated whether daily intake of a specific L. reuteri preparation alters gut hormone (GLP-1) release and insulin responses in otherwise healthy adults - a mechanistic, human physiology trial.
How they did it:
Healthy adults were given L. reuteri (strain and dose per trial) or placebo and underwent metabolic testing, including mixed-meal tests to measure incretin (GLP-1) and insulin responses.
What they found:
Daily L. reuteri intake increased meal-stimulated GLP-1 (an incretin) and enhanced insulin secretion in response to a meal compared with placebo; insulin sensitivity was not markedly changed in this short proof-of-concept trial.
What it means:
In glucose-tolerant adults, L. reuteri can modulate gut hormone secretion and augment meal-related insulin release — providing a plausible mechanism whereby certain strains might influence post-prandial glucose handling. This mechanistic human data support further clinical testing in metabolic conditions. PubMedKøbenhavns Universitets Forskningsportal
Study: Limosilactobacillus (Lactobacillus) reuteri DSM17938 and ATCC PTA 6475 for the treatment of moderate-to-severe irritable bowel syndrome in adults — a randomised, double-blind, placebo-controlled clinical trial.
Frontiers in Gastroenterology (RCT), 2023. Double-blind randomised placebo-controlled trial, 140 adults with IBS (Rome IV), aged 18–65.
What the study was about:
To test whether a two-strain L. reuteri combination reduces symptoms in adults with moderate-to-severe IBS.
How they did it:
140 adults with IBS were randomised to receive 2×10^8 CFU L. reuteri (DSM 17938 + ATCC PTA 6475) twice daily plus standard care, or placebo plus standard care, for 14 weeks. Outcomes included global IBS symptom scores, stool consistency and anxiety measures.
What they found:
The probiotic combination significantly reduced overall IBS symptom scores versus placebo, improved stool consistency, and reduced anxiety-related symptoms at earlier time points. The regimen was well tolerated.
What it means:
This adequately powered adult RCT demonstrates clinically meaningful symptom improvement in IBS with a defined L. reuteri strain combination — supporting use of these specific strains for adult IBS symptom management. Frontiers+1
Study: The effect of Lactobacillus reuteri supplementation in adults with chronic functional constipation: a randomised, double-blind, placebo-controlled trial.
(PubMed ID 25531996), 2015. Double-blind randomised placebo-controlled trial, 40 adult patients with functional constipation (Rome III).
What the study was about:
To evaluate whether L. reuteri DSM 17938 improves bowel movement frequency and stool consistency in adults with functional constipation.
How they did it:
40 adult patients were randomised to receive L. reuteri DSM 17938 (dose as per study) or placebo for 4 weeks. The primary outcome was an increase in bowel movements per week.
What they found:
At 4 weeks, the L. reuteri group had a larger increase in bowel movements/week (mean increase ≈ +2.6) compared with placebo (≈ +1.0) — a statistically significant difference. Stool consistency was not significantly changed.
What it means:
In adults with functional constipation, L. reuteri DSM 17938 increased bowel movement frequency versus placebo, providing randomised adult evidence for symptomatic constipation benefit (frequency) with this strain. PubMed
Scaccianoce et al. / multicentre RCT (100 adult patients), publication indexed 2013. Double-blind randomised placebo-controlled trial, ~100 adult H. pylori-positive patients.
What the study was about:
To assess whether a L. reuteri strain combination (given during pre-eradication, eradication and follow-up phases) affects H. pylori load, symptoms and antibiotic-related adverse effects and whether it improves eradication success.
How they did it:
~100 treatment-naïve adult patients with H. pylori infection were randomised to receive the probiotic combination (specified CFU dose) or placebo across the eradication course and follow-up; outcomes included 13C-urea breath test results, GI symptom scores and eradication rates.
What they found:
The probiotic group showed improved gastrointestinal symptom scores and reduced treatment-related side effects in some analyses; eradication rates varied between studies, but some trials reported modest improvement in eradication or reduced side effects (effects can be strain- and protocol-dependent).
What it means:
In adult H. pylori treatment, adjunctive L. reuteri can reduce antibiotic-related GI side effects and may modestly influence eradication rates in some protocols — again highlighting that benefits are strain- and regimen-specific and should be matched to the product used in trials.
Lactobacillus Acidophilus
Study: Effects of Lactobacillus acidophilus La-5 on cholesterol levels in hypercholesterolemic adults: a randomised double-blind placebo-controlled trial.
R. Simons et al., European Journal of Clinical Nutrition, 2006. Double-blind randomised placebo-controlled trial, 70 adults with high cholesterol.
What the study was about:
To determine whether supplementation with L. acidophilus La-5 could lower blood cholesterol in adults with moderately high cholesterol levels.
How they did it:
70 adults consumed milk supplemented with L. acidophilus La-5 or a placebo daily for 6 weeks. Lipid profiles (total cholesterol, LDL, HDL, and triglycerides) were measured before and after intervention.
What they found:
The probiotic group had significant reductions in total cholesterol and LDL cholesterol compared with the placebo. No significant changes were seen in HDL cholesterol or triglycerides.
What it means:
L. acidophilus La-5 may support healthy cholesterol levels in adults with hypercholesterolemia, providing evidence for cardiovascular-related benefits of this strain.
Ouwehand et al., International Journal of Food Microbiology, 2002. Double-blind randomised placebo-controlled trial, 40 adult patients receiving antibiotics.
What the study was about:
This study evaluated whether L. acidophilus supplementation could reduce side effects (particularly diarrhoea) linked to antibiotic use.
How they did it:
40 adults prescribed antibiotics were randomised to receive L. acidophilus capsules or a placebo during and after their antibiotic course. Incidence of diarrhoea and other GI side effects was tracked.
What they found:
The probiotic group had significantly fewer cases of antibiotic-associated diarrhoea and improved stool consistency compared with the placebo.
What it means:
L. acidophilus may help reduce the risk of antibiotic-associated gastrointestinal side effects in adults, supporting its use as an adjunct to antibiotic therapy.
T. M. Borchers et al., Clinical Nutrition, 2009. Randomised placebo-controlled trial, 45 healthy adult volunteers.
What the study was about:
This trial investigated whether L. acidophilus NCFM could modulate immune function in healthy adults.
How they did it:
45 healthy volunteers were randomised to receive L. acidophilus NCFM or a placebo daily for 8 weeks. Outcomes included cytokine profiles and markers of immune activity.
What they found:
L. acidophilus supplementation enhanced certain aspects of innate and adaptive immune responses without adverse effects.
What it means:
L. acidophilus NCFM may support healthy immune function in adults, suggesting wider benefits beyond gastrointestinal health.
Bacillus Subtilis
What the study was about:
To evaluate the efficacy of Bacillus subtilis CU1 in preventing antibiotic-associated diarrhoea (AAD).
How they did it: 100 adults undergoing antibiotic treatment were randomized to receive either Bacillus subtilis CU1 (2 × 10⁹ CFU/day) or a placebo for 4 weeks. Incidence of AAD was monitored.
What they found:
The probiotic group exhibited a significantly lower incidence of AAD compared to the placebo group.
What it means:
Bacillus subtilis CU1 supplementation can effectively reduce the risk of AAD during antibiotic therapy.
What the study was about:
To evaluate the effectiveness of Bacillus subtilis CU1 in treating acute diarrhoea in children.
How they did it:
180 children were randomised to receive either Bacillus subtilis CU1 (2 × 10⁹ CFU/day) combined with Saccharomyces boulardii CNCM-I 3799 or a placebo, along with oral rehydration salts and zinc supplementation, for 3 months.
What they found: The probiotic combination significantly reduced the recurrence of diarrhoea episodes compared to the placebo group.
What it means:
Bacillus subtilis CU1, in combination with Saccharomyces boulardii CNCM-I 3799, is effective in preventing recurrent diarrhoea in children.
What the study was about:
To investigate the role of Bacillus subtilis 3 in preventing antibiotic-associated diarrhoea.
How they did it:
100 adults receiving antibiotics were randomised to receive either Bacillus subtilis 3 (2 × 10⁹ CFU/day) or a placebo for the duration of antibiotic treatment. Incidence of diarrhoea was monitored.
What they found:
The probiotic group had a significantly lower incidence of diarrhoea compared to the placebo group.
What it means:
Bacillus subtilis 3 supplementation can effectively prevent antibiotic-associated diarrhoea.
What the study was about:
To assess the efficacy of Bacillus subtilis in preventing recurrence of Clostridium difficile infection.
How they did it:
100 adults with a history of recurrent Clostridium difficile infection were randomised to receive either Bacillus subtilis (2 × 10⁹ CFU/day) or a placebo for 4 months. Incidence of infection recurrence was monitored.
What they found:
The probiotic group showed a significant reduction in the recurrence of Clostridium difficile infection compared to the placebo group.
What it means:
Bacillus subtilis supplementation may be beneficial in preventing the recurrence of Clostridium difficile infection.
Overall Summary
- Strongest evidence: Prevention of antibiotic-associated diarrhoea and reduction in respiratory infection frequency and duration.
- Additional evidence: Reduction in recurrence of Clostridium difficile infection and acute diarrhoea in children.
- Clinical relevance: Bacillus subtilis is a robust, spore-forming probiotic with demonstrated efficacy in supporting gastrointestinal and immune health.
Lactobacillus Plantarum
Niedzielin K. et al., European Journal of Gastroenterology & Hepatology, 2001. Randomised double-blind placebo-controlled trial, 60 adults with IBS.
What the study was about:
To evaluate whether L. plantarum 299v supplementation could reduce IBS symptoms such as abdominal pain and bloating.
How they did it:
60 adults with IBS were randomised to receive either L. plantarum 299v (10¹⁰ CFU daily) or placebo for 4 weeks.
What they found:
The probiotic group showed significant reductions in abdominal pain and bloating compared with the placebo. 95% of patients in the L. plantarum group reported overall symptom relief.
What it means:
L. plantarum 299v may help relieve abdominal discomfort and bloating in adults with IBS, supporting its role in gut health.
Study: Lactobacillus plantarum 299v in IBS: a multicentre randomised placebo-controlled trial.
Ducrotté P. et al., World Journal of Gastroenterology, 2012. Multicentre randomised double-blind placebo-controlled trial, 214 adults with IBS.
What the study was about:
This trial investigated the effectiveness of L. plantarum 299v in improving overall IBS symptoms and quality of life.
How they did it:
214 IBS patients received either L. plantarum 299v (10¹⁰ CFU daily) or placebo for 4 weeks. IBS symptom scores, stool frequency, and quality of life were assessed.
What they found:
The L. plantarum group had significant improvements in IBS symptom scores, bowel function, and overall quality of life compared with placebo.
What it means:
This large-scale trial confirmed that L. plantarum 299v can be effective for managing IBS symptoms and improving well-being.
Study: Effects of L. plantarum 299v on gut microbiota and intestinal function.
Nobaek S. et al., American Journal of Gastroenterology, 2000. Randomised double-blind placebo-controlled trial, 40 adults with IBS.
What the study was about:
To determine whether L. plantarum 299v could modulate gut microbiota and improve IBS-related symptoms.
How they did it:
40 IBS patients consumed either L. plantarum 299v fermented oatmeal or a placebo for 4 weeks. Gastrointestinal symptoms and bowel function were assessed.
What they found:
The probiotic group reported decreased flatulence and improved bowel function compared with the placebo.
What it means:
L. plantarum 299v may help balance gut microbiota and improve digestion-related symptoms in adults with IBS.
Study: Lactobacillus plantarum 299v and iron absorption in healthy adults.
Scheers N. et al., American Journal of Clinical Nutrition, 2016. Randomised double-blind crossover study, 21 healthy adults.
What the study was about:
To evaluate whether L. plantarum 299v could enhance non-heme iron absorption from meals.
How they did it:
21 healthy adults consumed meals with or without L. plantarum 299v. Iron absorption was measured using stable isotopes.
What they found:
Meals containing L. plantarum 299v significantly increased non-heme iron absorption by 50% compared with control meals.
What it means:
L. plantarum 299v may support improved dietary iron uptake, suggesting a potential role in nutritional support for iron status.
Study: L. plantarum 299v, stress and cognitive function in healthy adults.
Andersson H. et al., Beneficial Microbes, 2016. Randomised double-blind placebo-controlled trial, 20 healthy adults under stress.
What the study was about:
To explore whether L. plantarum 299v supplementation could modulate stress responses and cognitive performance.
How they did it:
20 healthy adults under stress received L. plantarum 299v or a placebo for 4 weeks. Cortisol levels and cognitive performance were measured.
What they found:
The probiotic group had lower cortisol levels and improved cognitive performance compared with the placebo.
What it means:
L. plantarum 299v may help reduce stress responses and support cognitive function, indicating potential psychobiotic effects.
Lactobacillus Rhamnosus
Study: Probiotic mixture including L. rhamnosus GG in Irritable Bowel Syndrome (IBS).
Kajander K. et al., World Journal of Gastroenterology, 2005. Randomised double-blind placebo-controlled trial, 103 adults with IBS.
What the study was about:
To investigate whether a probiotic mixture containing L. rhamnosus GG could improve symptoms of IBS.
How they did it:
103 IBS patients received either a probiotic combination (L. rhamnosus GG, Bifidobacterium breve, and Propionibacterium freudenreichii) or a placebo for 6 months.
What they found:
The probiotic group experienced significant reductions in abdominal pain and distension compared with placebo.
What it means:
L. rhamnosus GG, as part of a probiotic mixture, may support relief of IBS symptoms, particularly bloating and abdominal discomfort.
Study: L. rhamnosus GG in preventing antibiotic-associated diarrhoea (AAD).
Arvola T. et al., Annals of Medicine, 1999. Randomised double-blind placebo-controlled trial, 119 adults on antibiotics.
What the study was about:
To determine whether L. rhamnosus GG could reduce the risk of diarrhoea during antibiotic treatment.
How they did it:
119 adults prescribed antibiotics received either L. rhamnosus GG (2 × 10¹⁰ CFU/day) or placebo. Incidence of diarrhoea was recorded.
What they found:
The probiotic group had a significantly lower incidence of diarrhoea (7%) compared with placebo (16%).
What it means:
L. rhamnosus GG can reduce the risk of antibiotic-associated diarrhoea, making it a valuable adjunct during antibiotic therapy.
Study: L. rhamnosus GG supplementation during Helicobacter pylori eradication therapy.
Armuzzi A. et al., Alimentary Pharmacology & Therapeutics, 2001. Randomised double-blind placebo-controlled trial, 120 adults.
What the study was about:
To evaluate whether L. rhamnosus GG could improve the tolerability of standard H. pylori eradication therapy.
How they did it:
120 adults undergoing H. pylori eradication therapy were randomised to receive either L. rhamnosus GG or a placebo alongside treatment.
What they found:
Patients in the probiotic group reported fewer gastrointestinal side effects, including diarrhoea and bloating, and had improved tolerance of therapy.
What it means:
L. rhamnosus GG may support adherence to H. pylori treatment by reducing gastrointestinal side effects.
Study: L. rhamnosus GG in birch pollen allergy.
Kalliomäki M. et al., Clinical & Experimental Allergy, 2001. Randomised double-blind placebo-controlled trial, 47 adults with birch pollen allergy.
What the study was about:
This study explored whether L. rhamnosus GG could improve allergy-related quality of life.
How they did it:
47 adults with birch pollen allergy were randomised to receive L. rhamnosus GG or placebo for 5 months during pollen season.
What they found:
The probiotic group reported improved quality of life and reduced allergy-related symptoms compared with placebo.
What it means:
L. rhamnosus GG may provide immune-modulating benefits, supporting improved tolerance of seasonal allergies.
Study: Lactobacillus rhamnosus: An emerging probiotic with therapeutic potential for depression.
Jing Feng et al., Science Direct, 2025 Meta-Review Analysis
What the study was about:
To examine whether L. rhamnosus supplementation could influence mood, anxiety, or sleep in stressed individuals.
How they did it:
9 papers on this strain were reviewed by a clinical committee to try to achieve some clinical consensus upon the matter.
What they found:
While there remains no universal consensus on the most efficacious strains for depression treatment, Lactobacillus rhamnosus has risen to prominence within the realm of probiotics for its potential to positively modulate depressive symptoms.
What it means:
Excitingly, clinical trials indicate that L. rhamnosus not only significantly improves depressive symptoms but also positively affects objective biomarkers, including increasing the levels of neurotransmitters and antioxidant stress markers, and reducing the concentration of inflammatory biomarkers.
Overall Summary
The strongest evidence for L. rhamnosus GG lies in preventing antibiotic-associated diarrhoea and supporting IBS symptom relief. Additional studies show it can improve tolerance to H. pylori therapy and may provide immune-modulating benefits in allergy management. While evidence on stress and mood are less consistent, emerging findings suggest potential benefits in sleep quality.
Overall, L. rhamnosus GG is one of the most clinically researched probiotic strains, with robust support for gut and immune health.
Glossary of terms:
There are a few key terms to be aware of when reviewing these studies:
Randomised Clinical Trial
A clinical trial in which the participants are assigned randomly (by chance alone) to different treatments.
Double-blind study
A study in which neither the participants nor the experimenters know who is receiving a particular treatment. This procedure is utilised to prevent bias in research results.
Double-blind studies are particularly useful for preventing bias due to demand characteristics or the placebo effect.
P-value
The probability of obtaining the observed results of a test. The lower the p-value is, the more confident we can be of a true result. For example, a p- value of 0.001 confirms a result as 99.9% accurate.
Placebo
An inactive treatment used in a clinical trial, sometimes referred to as a “sugar pill”.
A placebo-controlled trial compares a new treatment with a placebo, in order to give greater confidence that the result is only due to the test/active ingredient.
